Traditionally, conventional mechanisms used to produce minute linear displacements have included those mechanisms which incorporate plain or compound levers with large ratios of mechanical advantage, such as finely pitched screws, large ratio gear trains, or hydraulic pistons.
Differential screws have been, heretofore, used to further increase the ratio of movement reduction. Additionally, thermo-mechanical or piezoelectric translating drives requiring both the input of thermal or electrical energy with a supplementary coarse mechanical adjustment have been used for providing minute displacement.
To produce very fine, linear displacement, a relatively large movement must be converted to a small movement in order to provide the minute displacements needed for fine measurement. Just as there are limits to the amount of lever advantage available from a lever system, there is a limit to the amount of motion reduction achievable by a screw or gear mechanism. Inherent in such systems are unavoidable cumulative slack resulting in backlash or mechanical hysteresis. All interactive mechanical parts exhibit a degree of looseness which is manifested as backlash. Mechanical hysteresis arises during the interaction of materials or surfaces of gears and other machine components, where the strain on a material depends not only on the instantaneous value of the applied stress but also on the previous history of the stress applied to a material.
Also, heretofore, conventional minute displacement devices have provided one integrated reduction of movement, requiring additional separate structures for greater and more rapid displacements.
An example of a conventional use of a micrometer to provide coarse and fine adjustments includes U.S. Pat. No. 4,139,948 to Tsuchiya which is directed to a micromanipulator which is constructed to house a pair of micrometer heads (20 and 22 of FIG. 1) which together act against a differential lever 44 to provided fine adjustment. Coarse adjustment is obtained by the individual manipulation of the thimble 38 of micrometer head 22, while the principle of the differential lever or gears allows a 10:1 magnification of displacement between coarse and fine adjustment. Fine adjustment results from the manipulation of micrometer head 20. Thus, this patent teaches one structure for coarse adjustment and another for fine adjustment. Additionally, the interplay of many separate elements, for example, the slider block 48, in conjunction with the bottom plate 6, rear plate 12, and the side plates, may give rise to inherent backlash. A greater negative mechanical advantage needed for even finer adjustments than the 10:1 ratio taught in the U.S. Pat. No. 4,139,948 may be difficult to achieve due to this inherent backlash. Another disadvantage of the Tsuchiya patent is the arcuate movement of the differential lever 44 which leads to a non-linear relationship between the rotation of the fine micrometer head 20 and the movement of the slider block 48.
U.S. Pat. No. 4,331,384 to Eisler presents an optomechanical system for moving optical elements through a number of degrees of freedom. The system attempts to use a minimal number of basic elements to achieve linear and rotational movement. The elements are manipulated by a low friction lever mechanism with a high transmission ratio, through a micrometer screw, of a construction like that shown in U.S. Pat. No. 4,209,233 also to Eisler.
The present invention includes a novel application of a harmonic wave generator for use in conjunction with and superimposed upon a coarse adjustment micrometer to achieve a nanometric displacement drive system, and will be discussed later in this specification. The following are examples of prior art patents for improved wave generator mechanisms which may be of interest for understanding the level of skill in the art prior to the applicant's novel application of the harmonic wave generator in an improved nanometric drive system.
Anti-backlash devices, such as U.S. Pat. No. 3,020,775 to Musser, have heretofore been employed in connection with a harmonic wave generator, which operates as a motion reduction unit; however, the backlash addressed in the U.S. Pat. No. 3,020,775 is related to the control of backlash or play between mating gears, and no suggestion is made therein concerning incorporation of the harmonic wave generator into a nanometric drive mechanism.
U.S. Pat. No. 3,088,333 to Musser also provides an improved fluid wave generator which displays the use of a flex spline 22 (FIG. 1) affixed to a rotating output shaft 26. This patent is not, however, directed to a nanometric displacement drive as is the invention of the present disclosure.
Much of today's scientific experimentation requires minute movement in the ranges down to that of the wavelength of light or a fraction thereof. As the limits of motion reduction are reached by a conventional micrometer, the inherent backlash alone, present in conventional micrometers, will introduce unacceptable errors in measurements below the micrometric range. For example, one turn of a micrometer screw with 40 threads per inch provides a linear displacement of the micrometer shaft of 0.025 inches or 635 microns (10.sup.-6 meters). The spectrum of visible light ranges from 400 to 800 nanometers (10.sup.-9 meters, or 10.sup.-3 micron). Thus, one turn of the 40 thread per inch micrometer causes a linear movement of 635,000 nanometers. With a total range of 400 nanometers, it is not surprising that even with a small portion of a turn, it would be difficult to calibrate a conventional micrometer accurately anywhere within such a range. Even the entire range of the visible spectrum represents but 0.06% of one turn of the micrometer screw (0.06% of a turn is about 0.22 of one degree of arc). Even if one could guarantee a micrometer movement within a small linear displacement, the backlash inherent in the interplay of the micrometer screw in its nut would exceed the range of the visible light spectrum.
In order to provide linear displacement within the dimensional range of the visible spectrum wavelengths, it will be necessary for a mechanically based system to significantly improve the motion reduction ratio of the mechanism. This improvement must be provided without the need for cumbersome structures and, preferably, without the need to introduce thermal or electrical energy to drive such a mechanism.